1. FIELD OF THE INVENTION
The present invention relates to a sensor for detecting presence of low-molecular weight organic liquid such as gasoline and vapor thereof, and a detection system using this sensor.
2. DESCRIPTION OF THE PRIOR ART
Gasoline shares part of low-molecular weight organic liquid products, which is mainly used for the fuel of internal combustion engines, in particular, gasoline is mostly used for the fuel of automobile engines today. Normally, drivers refuel their automobiles at gasoline service stations. As it well known, gasoline is highly volatile and inflammable, and thus, extreme care must be taken while handling gasoline. As a matter of course, all the gasoline service stations strictly prohibit lighting of fire in the service site.
Conventionally, gasoline service stations store gasoline in underground tanks. Gasoline is supplied to the gas tank of each automobile by applying a refueling machine. Service men must very carefully supply gasoline to the automobile tank from the refueling machine. Gasoline leaked out of the refueling machine is quite dangerous because of potential fire hazard. In addition to a critical danger caused by leakage of highly volatile liquid, vapor thereof leads to a possibility of explosion when vaporized liquid concentration exceeds explosive limits. Therefore, leakage of highly volatile liquid should always be under strict observation.
The same applies to those low-molecular weight organic liquid products other than gasoline. To solve those problems mentioned above, one of the preceding arts proposes a sensor for detecting leakage of a low molecular weight organic liquid product such as gasoline. The proposed sensor is provided with an electric conductive sheet made from cross-linked organic material mainly composed of natural rubber and/or ethylene-propylene (EP) rubber. The sensor also has terminals which are provided on both ends of the conductive sheet.
Current constantly flows through the conductive sheet. If either low molecular weight organic liquid such as gasoline or vaporized content adheres to the conductive sheet, the affected portion of the conductive sheet swells to cause increase of electric resistance of the conductive sheet, and as a result, current cannot easily flow through the conductive sheet. The proposed sensor electrically detects such state and in turn detect presence of leaked low molecular weight organic liquid.
Normally, a sensor of this type is installed inside of an internal pipe of a refueling machine. However, responsivity of electric resistance of the conductive sheet against low molecular weight organic liquid and vaporized content is not very high, electric resistance of the conductive sheets does not readily rise even when either liquid or vaporized content of a low molecular weight organic product adheres to the conductive sheet.
On the other hand, when a sensor provided with a water-repellent film is exposed to highly-humid atmosphere for a long time, water or moisture may eventually reach the conductive sheet. If water or moisture adheres to the conductive sheet through the water-repellent film, it not only prevents the conductive sheet from swelling by the effect of adhered liquid or vapor of a low molecular weight organic product, but it also causes the sensor itself to malfunction as a result of the increased contact resistance between terminals and the conductive sheet due to corrosion of terminals by electrolysis.
As is clear from the above description, the conventional sensor is not only poor in sensitivity against leakage of low molecular weight organic liquid or vapor, but it also lowers responding characteristic due to adhesion of water or moisture, and thus, it cannot precisely detect leaked organic liquid or vapor. Because of this disadvantage, such a conventional sensor mentioned above is actually rarely used.
As the situation stands now, no measure is taken to effectively prevent leakage of low molecular weight organic liquid such as gasoline and vapor thereof particularly in gasoline service stations today.